This invention relates generally to the manufacture of large active-matrix liquid crystal displays, and more specifically to a method and apparatus for forming a silicon film across the surface of a glass substrate.
Many common electronic devices utilize liquid crystal displays (LCDs) to display images or characters. LCDs have become increasingly popular because they offer several advantages over other display technologies. For example, LCDs are typically thinner, lighter, and consume much less power than cathode ray tube (CRT) monitors. Additionally, LCDs do not suffer from convergence issues, and images may be displayed at low refresh rates without suffering image flicker problems.
Common-plane-based LCDs are generally used in simple displays that repeatedly show the same images. For example, common-plane-based LCDs are commonly used as digital watch and microwave timer displays. Computers, televisions, and other such devices requiring complex display capabilities generally utilize either passive matrix or active matrix LCDs.
Passive-matrix LCDs employ a simple grid to supply a charge to a particular pixel on the display. The grid is typically formed by applying a transparent conductive material, such as indium-tin oxide, to two glass substrates. On one substrate, the transparent conductive material is formed in columns; on the other substrate, the transparent conductive material is formed in rows. The rows and columns are connected to integrated circuits that control when a charge is sent down a particular column or row. A liquid crystal material is sandwiched between the two glass substrates, and a polarizing film is added to the outer side of each substrate. To turn on a pixel, an integrated circuit sends a charge down the correct column of one substrate and a ground activated on the correct row of the other substrate. The row and column intersect at the designated pixel, and the voltage field across the intersection area causes the liquid crystals at that pixel to untwist.
Passive-matrix LCDs may have significant drawbacks. For example, the response time, or the ability of the LCD to refresh a displayed image, is typically slow. As a result, rapidly changing screen content, such as video or fast mouse movements, may cause xe2x80x9csmearingxe2x80x9d because the display cannot keep up with image content changes. Additionally, voltage control may be imprecise, hindering a passive matrix LCD""s ability to control individual pixels without affecting adjacent pixels. For example, voltage applied to untwist one pixel may cause surrounding pixels to partially untwist, resulting in fuzzy images, poor contrast, and ghosting of off pixels in the same rows and columns. Moreover, increased screen-size and pixel counts negatively affect display parameters due to large capacitance and limited conductivity of the electrodes.
In an active-matrix LCD, pixel addressing take place behind the liquid crystal film. The rear surface of the front substrate is coated with a continuous transparent electrode while the rear surface is patterned into individual pixels. Thin film transistors (TFTs), comprising tiny switching transistors and capacitors, are used to apply a voltage to a particular pixel on the display. The TFTs are typically formed in a matrix on a quartz or glass substrate. To address a particular pixel, the proper row is switched on and a charge is sent down the correct column. Because all of the other rows that the column intersects are turned off, only the capacitor at the designated pixel receives a charge, and the capacitor is structured to hold the charge until the next refresh cycle. As a result, the use of TFTs eliminates the problems of slow response speed and ghosting that afflicts passive-matrix LCDs. Additionally, active-matrix LCDs may be used to create a greater range of gray scale by controlling the amount of voltage supplied to a particular pixel in additional increments. For example, active-matrix LCDs can offer 256 or more levels of brightness per pixel. Due to enhanced performance benefits, active-matrix LCDs are currently used in almost all portable electronic device applications with diagonal screen sizes from about 2-15 inches.
Each pixel within a color active-matrix LCD is formed from three sub-pixels with red, green, and blue color filters. Each sub-pixel is connected to a TFT that controls the degree of liquid crystal action at that sub-pixel. As a result, the intensity of the red, green, and blue elements of each pixel forming an image on the LCD may each be independently adjusted by controlling the amount of voltage applied to each sub-pixel. For example, by controlling the TFT voltage output to each sub-pixel, the intensity of each sub-pixel may be varied to produce over 256 shades. Combining the sub-pixels produces a possible palette of approximately 16.8 million colors. Such displays require a large number of TFTs. For example, a typical laptop computer with an active-matrix color LCD supporting resolutions up to 1,024xc3x97768 requires 2,359,296 TFTs.
In the past, TFTs have typically been formed from amorphous silicon (a-Si) deposited by plasma enhanced chemical vapor deposition (PE-CVD) processes. The advantage of using amorphous silicon is that it may be formed at lower process temperatures on relatively inexpensive glass substrates. However, amorphous silicon TFTs suffer from numerous known limitations.
For example, high electron mobility is a critical factor in increasing LCD performance. However, the limited electron mobility inherent to amorphous silicon TFTs provides for limited frame refresh rates and pixel densities. Higher amorphous silicon TFT performance may be achieved using powerful driver circuitry, but the resulting high energy consumption is detrimental to portable electronic device functionality.
One of the largest cost elements in an active-matrix LCD is the external driver circuitry. In a typical active-matrix display utilizing amorphous silicon TFTs, each pixel is independently connected to discrete logic chip drivers arranged on printed circuit boards (PCBs) around the periphery of the display area. As a result, a large number of external connections are required from the LCD panel to the PCBs, resulting in high manufacturing costs. The arrangement of PCBs around the periphery of the display area also limits the form factor of the casing surrounding the LCD.
The brightness of an LCD is determined in part by the aperture ratio of the pixels, or the ratio of light passing through each pixel to the entire area of the pixel and associated electronics. A larger aperture ratio allows more light to pass through the pixel, resulting in a brighter image on the LCD. Typical amorphous-silicon active-matrix LCDs have a pitch of approximately 0.3 mm with a corresponding color sub-pixel size of 0.1 mm or 100 microns. The TFTs and wire connections on these displays typically occupy corridors approximately 10 microns wide. Additionally, the properties of amorphous-silicon TFTs change when the material is exposed to heat and light, and amorphous-silicon TFTs must be shielded from ambient light to prevent instabilities, resulting in a reduced aperture ratio. Consequently, active-matrix displays utilizing amorphous silicon TFTs often require more powerful backlighting, resulting in increased energy consumption.
Crystalline forms of silicon, such as poly-silicon (p-Si) and mono-crystalline silicon, have higher electron mobilities than amorphous silicon. As a result, increased frame refresh rates, higher pixel densities, and larger aperture ratios may be achieved with TFTs formed from crystalline silicon materials. Additionally, the use of crystalline silicon TFTs may allow the driver circuitry and peripheral electronics to be made an integral part of the LCD itself, thereby reducing the number of components required to manufacture an individual display and allowing larger LCDs to be fitted into existing casing designs.
At the present time, poly-silicon TFTs have been developed for use in small active-matrix projection LCDs. Poly-silicon may be directly deposited by a chemical vapor deposition (CVD) process at temperatures greater than 590xc2x0 C. Due to the high process temperatures, poly-silicon deposition typically requires the use of expensive quartz substrates. Metal induced crystallization is able to overcome some of these problems, resulting in a lower process temperature of approximately 450xc2x0 C.
Small projection LCDs utilizing poly-silicon TFTs formed on quartz substrates may be manufactured economically due to their small size, typically less than two inches diagonally. However, the cost of quartz substrates increases exponentially with size, and the use of one-piece quartz substrates to manufacture larger, direct-view LCDs is prohibitively expensive.
Currently, some manufacturers are attempting to form poly-silicon on large conventional glass substrates by means of thermal conversion of amorphous silicon using Excimer lasers. Using this process, n-type mobilities of 10-500 have been achieved, equaling those of crystalline silicon devices. However, it is unlikely the Excimer laser thermal conversion process can adequately enhance p-type poly-silicon mobilities. As a result, this process fails to achieve the necessary p-type mobilities required for CMOS devices, such as SRAM components in the LCD external driver circuitry.
Other manufacturers have developed processes whereby two or more active-matrix LCDs are joined together to form a xe2x80x9ctiledxe2x80x9d display device. Several different approaches have been applied to this methodology. In one approach, seams between individual display tiles are deliberately made visible and displayed images are extended continuously over the tiles and seams. This approach is used by Clarity and Pioneer, among others, in their stackable video-wall display products. In another approach, seams between individual display tiles are hidden such that they are completely invisible to the naked eye under normal viewing conditions. Rainbow Displays Incorporated located in Endicott, N.Y. has developed a process by which two or more active-matrix LCDs with pixel pitches less than 1 mm may be xe2x80x9cseamlesslyxe2x80x9d joined to form a single tiled display device. However, seamless tiled displays face significant manufacturing challenges. For example, it is extremely difficult to maintain continuous pixel pitch across seams and tile alignment must be extremely precise to meet visual acuity standards. Furthermore, uniform tile luminance and chromaticity must be maintained across seams. As a result, seamless tiled displays typically require sophisticated light management and digital signal processing techniques.
Consequently, a need exists for a low-temperature process of forming crystalline silicon, such as poly-silicon and mono-crystalline silicon, on conventional glass substrates.